Manufacturing sheet metal parts generally begins with a blanking operation where blanks are cut from coils of rolled steel or aluminum. The outer perimeter and large openings may be trimmed to form a blank that is then formed in subsequent drawing, flanging, punching, piercing and hemming processes. One example of a part that is blanked with a large opening is a body side panel that spans the side of a vehicle and defines the opening that receives the doors in a nesting relationship. One problem with forming precut openings in a blank is that in subsequent forming operations the material around the opening must flow from both the inside of the opening and the areas outside the opening. The precut opening is expanded when the blank is formed and may form splits in the material around the opening.
With the need to reduce the weight of vehicles to meet fuel economy standards, the development of sheet metal parts made from aluminum or high strength alloys is increasing. Aluminum and high strength steel are less malleable than mild steel and the problems relating to splitting at the inner perimeter of larger openings is more prevalent. Small cracks or small imperfections in the cut edge formed when aluminum blanks are cut out expand to form splits in a subsequent forming operations because additional metal is drawn from the inside of the opening.
The root causes of splitting at the inner edge of the blank during draw die expansion in subsequent forming operation originates from excessive strain hardening and small imperfections due to rough fracture surfaces, micro-cracks, burrs, and gall marks. Excessive strain hardening and imperfections severely limit the expansion capacity of the metal. Avoiding problems relating to splits caused by imperfections in precut openings in aluminum panels limits opportunities to use aluminum sheet metal parts. Panel splits may cause substantial yield losses in the stamping process due to the need to scrap parts that have splits in critical areas. In addition, problems relating to splits in panels result in cost overruns, supply shortages, potential quality problems, and reduced manufacturing line availability.
This disclosure is directed to solving the above problems and other problems as summarized below.